Process for bleaching, refining, and cottonizing raw cellulose



Patented Sept. 29, 1936 UNITED STATES PROCESS FOR BLEACHING, REFINING, AND

- COTTONIZING RAW CELLULOSE Francesco Carlo Palauo and Fortunato Palazzo, Florence, Italy No Drawing. Application May 4, 1934, Serial No.

Cellulose experts know very well the many dimculties of treatment of raw cellulose when aiming at perfect snow-white bleaching and obtaining a comparatively high content of alpha-cellulose without sensible decrease of yield and without impairing of solidity of fibres. If woodcellulose with a high percentage of alpha cellulose as hitherto produced have not yet amrmed themselves in the artificial silk trade and kindred branches, this is mainly due to the fact that, even if faultless, they cannot be produced at convenient prices both owing to the high cost of the treatments in themselves and the comparatively poor yield obtained from raw celluloses.

On the other hand also the simple bleaching of celluloses even if carried out without aiming at a higher than the usual percentage of alpha cellulose (87%-89% on dry material) .involves a rather high cost of production when the bleached product is to satisfy the particularly severe requirements of the viscose industry.

With the invention hereinafter illustrated in its characteristic features and described in its details the technical and economical disadvantages with which modern purifying methods are more or less affected, are completely removed, because, as a matter of fact, the high bleaching degree and the more or less high percentage of alpha cellulose are conciliated with a notable solidity of fibre and satisfactory yield of finished product and moderate cost of production due both to the high yields and small consumption of reactives and rather short duration of treatments.

Salient features of invention The salient features of invention consist in the combination of advantages hitherto deemed, at least in part, incompatible with each other, and obtained both by rational coordination of treatments and adapting their intensity to the special final product to be obtained which, even if excellently bleached, may reach different refining degrees according to whether it is designed for paper manufacturing or for artificial silk, or whether it is to be used with success as a surrogate of cotton linters in the manufacture of nitrocellulose, acetyl cellulose, etc.

Refining of raw wood celluloses owing to the rather energetic methods and chemical reagents with which it has been hitherto effected must necessarily involve, even in the most favorable cases, a heavy shortage, most certainly disproportioned to the lignine and hemicellulose percentage contained in the raw celluloses. The

In Italy February 12, 1934 5 Claims. (Cl. 92-9 present invention however is very neatly and characteristically, difierentiated from the bleaching and refining processes hitherto used by the fact that the complete removal of lignine and its derivatives such as absolutely necessary for ob- 5 taining a durable snow-white colour is not inseparably bound to the simultaneous removal of a considerable part of the hemicellulosic substances, whilst, owing to the efficient action of the reagents chosen, under certain conditions, 10 very high alpha-cellulose percentages with satisfactory yields may be obtained. I

Besides refined bleached cellulose with 90%--92% of alpha cellulose, which may find a large outlet in the paper and viscose silk industry 15 in competition with the best bleached celluloses of the world market, the processes according to invention, as a matter of fact, permit of obtaining other types of products, also completely free from lignine, but with gradually decreasing hemi- 2o cellulose percentages. The highest form of refining culminates, in this sense, in "cottonization furnishing a product which owing to its high percentage of alpha-cellulose (96%-97%), its very small content of hemicellulosic substances, its insignificant ash percentage (0,l60,12%) and ether soluble substances (0,12%-0,08%) its small copper figure (below 1) and the complete absence of lignine may successfully replace cottonlinters in all of the uses of the latter in the nitro- 30 cellulose, acetyl-cellulose and cupro-ammoniacal silk industries.

From these few remarks it will be easily understood that the broad possibilities of this invention are also reflected on the industrial economy 35 of the treatments. In fact, the gradual increases of cost of production in the scale of bleached refined cellulose to cottonized cellulose occur with suilicient proportionality to the obtained refining degree, so that also the higher cost of 4 production of cottonized cellulose with respect to that of bleached refined cellulose for. super viscose is largely made up by the vast and more remunerative possibilities of use of the new product. 4

As regards the treatments and their coordination, it is, before all, not superfluous to point out that the starting raw material whatever the degree of refining to be reached, is not submitted to any hot or cold preliminary treatment with 5 alkaline substances. The first phase of the process consists in any case in treating the raw cellulose with chlorine water. Under condition which will be indicated later on, this reagent directs its action selectively on' the lignine, and therefore 2 the greater part of the latter may be easily removed by eflecting, after the action oi! the chlorine water, a particularalkaline sulphitic treatment (second stage of process) which, without in any way aifecting the cellulose, brings the chlorine derivatives of the lignine finally in solution. The very small quantity of lignine still adhering to the fibres after this treatment is, at last, removed by the bleaching operation proper (third stage of process), which in fact, gives the final product a splendid snow-white appearance.

But whilst the bleaching operation,-distinguished according to invention by special characteristical features oi! their own, remains in every case the same whichever the type of refined product to be manufactured, the alkaline sulphitic treatment interposed between the attack with chlorine water and the bleaching operation, varies as to duration and intensity according to the degree of refining aimed at. Thus, for instance, when it is desired to obtain bleached refined cellulose the alkaline-sulphitic liquor contains no free alkali, and the treatment is effected at a temperature below C. If, on the other hand, cottonization is to be obtained, the treatment is designed for removing, together with the chlorine-derivatives of lignine, also the greater part of the hemlcellulosic substances, and consists in cooking at temperatures above 100 C. and with a different lye which, instead of sodic carbonate, contains sodic hydrate.

Practical industrial application The practical industrial application of the processes offers no difliculty at all, nor are special apparatuses required therefor.

The processes, furthermore, are very flexible so as to adapt the characteristical features of the products, as well as their respective costs, to the more or less refined applications for which the said products are designed. They also permit the use, as raw material, of the most varied qualities of cellulose, from those raw ones of 'good quality and easy bleaching to indifferent and hard ones containing a comparatively high percentage of lignine.

The chlorine water for the initial treatment is used at an average concentration of 0,30%- 0,40% at which it is easy to be obtained from liquid or gaseous chlorine without resorting to complicated and costly apparatuses. At this concentration it is to be added to the raw cellulose pulp which happens to be already reduced to a density of about 8% on the dry material, and added in such volume as to have 2-3 parts of chlorine on 100 parts or raw cellulose supposed to be air-dried, whilst the concentration of cellulose is thus lowered to 5%-6%. The duration of contact of the material with the reagent used to be from 3 to 4 hours, but it is obvious that it does not affect the substance of the process if according to the nature of the cellulose containing more or less lignine, or according to the way of carrying out the treatment with stagnant or circulating bath. its duration be limited to say 2 hours, or extended to 5 hours. Anyhow, the above indicated relations may vary within certain limits without substantially affecting the process, whilst as regards the quantity of chicrine it may be established as a rule that the action or the reagent is to be strictly confined to the chlorinating stage of lignine without extending to the bleaching oi cellulose.

When chlorinating is completed and a portion of liquid removed by natural dripping in vats with perforated bottoms, the cellulose pulp after a couple of washings is furthermore deprived of a great part of water by a centrifugal hydroextractor or suitable helical press and then subjected to the treatment with the alkaline sulphitic liquor the composition of which varies according to the final article to be manufactured.

In order to obtain bleached refined cellulose" a lye containing ),4% of anhydrous sodium sulphite and 0,4% of anhydrous sodium carbonate is used in the ratio of 8: 1 with respect to the raw cellulose supposed to be air-dried. The treatment of the product with this liquor is eilected in open vessels with indirect heating at a temperature of 80-90 C. and is usually completed in 1 hour. a

In order to obtain refined cellulose for super viscose" with 92-94% of alpha-cellulose a lye containing 0,5% of anhydrous sodic sulphite, 0,5% anhydrous sodium carbonate and 0,l% oi. sodium hydrate is used, also at the ratio of 8:1 with respect to the raw cellulose supposed to be air dried. However in this case the alkaline-sulphitic cooking is effected in boilers provided with pumps for the circulation of the liquid under a pressure of 1 atm. and for V hour.

If, in fine, cottonized cellulose of the said features is to be obtained, a lye of different composition is used which besides 0,5%-0,75% of anhydrous sodic sulphite contains from 0,75% to 1,25% of sodium hydrate. The proportion to be kept between cellulose supposed to be air-dried and lye is still 1:8 but the cooking conditions are somewhat different, the cooking being eifected at a pressure of 2 atm. and extended to 2-3 hours. Also in this case slight oscillations of these average values,eventually due to the nature of the raw cellulose or the special type of boiler,--obvi- (l ously do not alter the substance of the invention.

At the end of cooking the cellulosic material is thoroughly washed in vats or boilers and then subjected to bleaching. The latter is efiected by using a very much diluted solution (1%) of permanganate of potassium in a solution slightly acidulated with sulphuric acid (2%) which, even if diluted by the considerable quantity oi. water with which the cellulose happens to be mixed acts with particular rapidity selectively on the quantity of lignine still adhering to the fibres but leaves the cellulose completely unafiected owing both to the high degree of dilution and the short duration of contact. The slight excess of permanganate and the manganese oxide deposed on the fibres are easily decomposed by subsequent treatments of short duration and at ordinary temperature, with aqueous solution of sulphurous anhydrid which at the same time removes from the cellulose a large part of the ashes.

As is usual in the cellulose industry, bleaching is effected with circulating bath followed by thor oughly washing with water, whereafter the product is centrifugated in order to be reduced to fiakes, or passed on endless working machines for reducing it to boards or thin sheets.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:

1. Process for bleaching, refining and cottonlizing raw celluloses, characterized in that the raw cellulose to be treated is first subjected to the chlorinating action of chlorine water, the quantity of reagent being adapted to the lignine content of the.cellulose, said chlorinated cellulose material is then further subjected to a hot treatment with an alkaline sulphitlc lye selected from a group consisting of, first, a mixture of sodium sulphite with sodium carbonate in a 0.4% concentration each, second, a mixture of the said two salts in a 0.5% concentration with sodium hydrate in a 0.1% concentration, third, a mixture of sodium sulphite in a 0.5 to 0.75% concentration with sodium hydrate in a 0.75 to 1.25% con.- centration; the treatment lasting 1 to 3 hours at a temperature between and and a pressure between 1 and 2 atmospheres.

2. Process for bleaching, refining and cottonizing raw celluloses, characterized in that the raw cellulose to be treated is first subjected to the chlorinating action of chlorine water, the quantity of reagent being adapted to the lignine content of the cellulose, said chlorinated cellulose material is then further subjected to a hot treatment with an alkaline-sulphitic lye which contains sodium sulphite and sodium carbonate in a 0.4% concentration each, the treatment lasting from 1 to 3 hours at a temperature between 80 and 100 C., corresponding to pressures of l to 2 atm.

3. Process for bleaching, refining and cottonizing raw celluloses, characterized in that the raw cellulose to be treated is first subjected to the chlorinating action of chlorine water, the quantity of reagent being adapted to the lignine content of the cellulose, said chlorinated cellulose material is then further subjected to a hot treatment with an alkaline sulphitic lye consisting of a mixture of sodium sulphite and sodium carbonate in a 0.5% concentration each with sodium hydrate in a 0.1% concentration; the treatment lasting 1 to 3 hours at a temperature between 10 and 100 and a pressure between 1 and 2 atm.

4. Process for bleaching, refining and cottonizing raw celluloses, characterized in that the raw cellulose to be treated is first subjected to the chlorinating action of chlorine water, the quantity of reagent being adapted to the lignine content'of the cellulose, said chlorinated cellulose material is then further subjected to a hot treatment with an alkaline sulphitic lye consisting oi a mixture of sodium sulphite in a 0.5 to 0.75% concentration with sodium hydrate in a 0.75 to 1.25% concentration; the treatment lasting 1 to 3 hours at a temperature between 80 and 100 and a pressure between 1 and 2 atm.

5. Processes as claimed in claim 2, in which the cellulosic material from the alkaline sulphitic treatment is bleached with a solution of potassic permanganate slightly aoidulated with sulphuric acid and ultimately deprived of the greater part of the mineral substances by treatment with a diluted solution of sulphurous acid.

FRANCESCO CARLO PALAZZO. FORTUNATO PALAZZO. 

